1. Field of the Invention
The present invention relates to an opto-electronic scale reading apparatus used to measure displacement of one member relative to another.
2. Description of Related Art
One such type of a apparatus is known from GB 1,504,691 which discloses a readhead which is movable relative to a reflective scale, and which illuminates the scale with light via an index grating (the light being substantially perpendicular to the scale), to generate a periodic light pattern in the plane of the said index grating. The index grating thus serves as an analyser grating, and relative movement between the scale and the readhead results in a light modulation at the analyser grating. Also disclosed in this document is the generation of moire fringes by skewing the lines of the index grating fractionally with the respect to the direction of the lines of the scale. Such an arrangement is susceptible to differential contamination of the scale (the propensity of different regions of the scale to interact in a different way with a given beam of incident light), and so is inherently inaccurate.
To overcome this problem it has been proposed to provide an auxiliary grating, or other beam splitting means, up-beam of the analyser grating in order to generate plurality of phase-shifted light intensity modulations at the analyser grating. Such an arrangement is described for example in our co-pending international application WO 89/05440. However, such an arrangement is unsuitable for the situation in which the readhead is movable over a large distance (i.e. greater than 10 mm) relative to the scale in a direction perpendicular to the plane of the scale since, in the above described arrangement, the light is incident upon, and reflected off the scale at an angle. Movement of the readhead relative to the scale perpendicular to the plane of the scale changes the angle of incidence of the light on the scale detected by a detector, and thus requires a re-adjustment of, for example, the optics used to focus light onto relevant photo-detectors.